The invention relates generally to a system and method for delivering droplets on a substrate. More particularly, the invention relates to aligning nozzles in ink-jet print heads with respect to target points on a substrate.
One conventional type of printer forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land on the medium. Such a printer can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate control hardware and software. A conventional print cartridge for an inkjet type printer comprises an ink containment device and a fingernail-sized apparatus, commonly known as a print head, which heats and expels ink droplets in a controlled fashion. Other conventional inkjet type printers use piezo elements that can vary the ink chamber volume through use of the piezo-electric effect to expel ink droplets in a controlled fashion, e.g., the ink ejection device of Asai as described by U.S. Pat. No. 5,764,247. Piezoelectric nozzles are not typically implemented in cartridges, but instead are provided as nozzle arrays.
In some applications, more than one inkjet print cartridge will be designed into a printer. Usually this multiple print cartridge assembly is created to accommodate multiple colors of ink. Properly controlling the arrangement of various droplets of ink of different colors will result in a wide spectrum of perceivable colors. The clarity and quality of the resultant image is affected by the accuracy of the placement of the ink droplets on the medium. Printers that use multiple print cartridges, or alternatively an array of nozzles, to cooperatively form a single image usually require mechanical or electronic adjustment so that ink droplets printed by one nozzle align at precise locations on the receiving medium relative to those printed by another nozzle in the printer.
Cartridge-to-cartridge alignment has been eliminated in some printers with the use of a single multi-color ink cartridge having a print head employing three sets of orifices arranged in a group and receiving one color of ink for each group on the print head. Such a single multi-color print cartridge is inherently self-aligning due to the precise positioning of one set of orifices relative to another on the single orifice plate on the multi-color print cartridge. Even for this cartridge, however, unless other compensation is made, the orifice plate of the print head should be oriented precisely perpendicular to the direction of travel for accurately printed results.
Mechanical alignment of print cartridges is simple but expensive, requiring precision features created in the orifice plate of the print head, precision alignment of the cartridges during manufacture to alignment structures or secondary milling of alignment structures or adjustment within the printers cartridge carriage. In each of these foregoing implementations, there are stringent requirements on the printer and the cartridge carriage for either precision during manufacture and long-term stability, or complex adjustability and human intervention. Electronic alignment typically requires printing ink droplet dots on a separate region of the medium, scanning the medium with a detector for these dots, and then establishing time delays within the printer to compensate for the measured offsets. Again, printer complexity or human intervention and judgment are required to optimize this form of alignment.
In U.S. Pat. No. 5,448,269, Beauchamp et al. use a test pattern for multiple ink-jet cartridge alignment for bi-directional printing.
In U.S. Pat. No. 5,451,990, Sorenson et al. use specified test patterns as a reference for aligning multiple ink-jet cartridges.
In U.S. Pat. No. 5,600,350, Cobbs et al. teach multiple ink-jet print cartridge alignment by scanning a reference pattern and sampling the same with reference to a position encoder.
The method of aligning a print head provided by U.S. Pat. No. 6,193,350 by Hadley requires the creation and use of a delay time to solve the problems associated with dynamic alignment of the print head to multiple sequential targets.
Another prior art alignment technique includes mounting the print head in a fixed mechanical position. The print head is mounted to a mechanical frame which itself is mounted to the printing platform. The print head position including the position of the nozzles is known to the limit of the mechanical tolerances. The tolerances of the mechanical mounting can easily exceed the maximum tolerances required for precise ink jet printing for organic light emitting devices (typically 1-10 micrometers). The position of the nozzles within the mechanical frame of the print head can vary due to manufacturing tolerances of the print head.
The alignment system of U.S. Pat. No. 6,234,602 requires the printing of a test pattern according to a first data set, acquiring a second data set representative of the test pattern, fitting a first waveform representative of the first data set to the second data set to determine an initial fit offset value, partitioning the second data set into third data sets, fitting a measuring construct to each of the third data sets, and calculating an actual print head alignment offset value for each of the third data sets using the initial date offset with comparison data representative of comparing the measuring construct and the second data set.
Ink-jet printing has become popular as a deposition technique for the polymer, or small molecule, solutions used in organic electronics, such as organic light-emitting devices (OLEDs). One key challenge for this technique is the delivery of the solution droplets to target points of the substrate with a precision of approximately 1 to 10 micrometers. To meet this challenge, nozzles of the multi-nozzle print heads are aligned with great precision to the absolute position and the rotational angle of the substrate.
For some print head arrangements, as shown in FIG. 1, the array print head 10 is rotated with respect to the print direction (v) 12, in order to achieve a print pitch (pv) 14 different from the xe2x80x9cphysicalxe2x80x9d pitch (p) 16 of the print head. Such arrangements enable a print head 10 having nozzles 18 disposed at predetermined spaced intervals to deliver droplets for a variety of target 20 separations. Unfortunately, the rotation of the print head 10 with respect to the print direction 12 renders the problem of angle alignment of the print head to the substrate targets 20 even more difficult.
But, the advantages created by this angle alignment frequently justify the effort. One advantage is that print heads 10 rotated relative to the print direction 12 enable easy adjustment of nozzle 18 spacing to compensate for thermal expansion of the substrate. Note also that a wide variety of target 20 separations can be accomplished by using different patterns of nozzles 18 disposed on a print head 10; e.g., every other nozzle, or every third nozzle.
As shown in FIG. 2, for many prior art systems the substrate 25 is located on a sample chuck and kept in position by vacuum suction or by using a mechanical clamp. For droplet deposition, either the substrate 25 is positioned on a substrate stage 27 and moved underneath the print head 10 while the print head position remains fixed, or the print head is mounted on a stage and is positioned over the substrate while the substrate is kept at a fixed position. The problem of adjusting the angle of the print head 10 and the substrate 25 rotation is described in greater detail below with reference to FIG. 4.
An automatic inkjet nozzle inspection system from ImageXpert Inc. in Nashua, N.H. acquires an image of the nozzles, but does not determine the orientation of the nozzles or nozzle plate. The ImageXpert allows for determination of the nozzle plate rotation relative to the screen of their system, but not relative to the substrate (medium).
A first aspect of the invention is implemented in embodiments that are based on a system for delivering droplets on a substrate. The system comprises nozzles adapted to deliver droplets to target points on the substrate, a first camera, and means for determining an angular rotation of the substrate in a coordinate system. In some embodiments the means for determining the angular rotation includes a second camera. The substrate has a top surface and alignment marks. The nozzles have droplet delivery openings oriented towards the substrate for delivery of the droplets. The first camera is oriented substantially towards the nozzle openings, and the second camera is oriented substantially towards the substrate top surface.
A second aspect of the invention is implemented in embodiments that are based on a method for delivering droplets on a substrate. The method comprises providing nozzles and obtaining an angular displacement value in a coordinate system. The nozzles include a first nozzle and a second nozzle. The nozzles are disposed in proximity of the substrate. The angular displacement corresponds to at least two alignment marks where the alignment marks are disposed on the substrate. The method further comprises sequentially placing the first nozzle and the second nozzle in a position in a viewing area of a first camera, registering sets of coordinates in the coordinate system, and determining an angular rotation of the first nozzle and the second nozzle in the coordinate system. The first nozzle and the second nozzle each have at least one opening for the delivery of droplets. The first camera is oriented substantially towards the nozzle openings.
A third aspect of the invention is implemented in embodiments that are based on a method for assessing the suitability of a print head having at least two nozzles for manufacturing. The print head has at least a first nozzle and a second nozzle. The first nozzle has at least one droplet delivery opening. The method comprises obtaining an angular displacement value in a coordinate system, sequentially placing the first nozzle and the second nozzle within a viewing area of a first camera, registering sets of coordinate data in the coordinate system, and comparing the sets of coordinate data and the angular rotation data to predetermined suitability values. The alignment marks disposed on a substrate. The first camera faces substantially towards the nozzle openings. Each set of coordinate data is associated with one of the nozzles. The comparing step is performed within a system for delivering droplets.
These, and other, aspects of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following description, while indicating preferred embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such modifications.